An example of a conventional light-emitting device is described in U.S. Pat. No. 9,525,148 (Kazlas et al., issued Dec. 20, 2016). FIG. 1 is a drawing depicting an exemplary representation of such a conventional light-emitting device. A conventional light-emitting device includes an anode 104 and cathode 100, and a light-emitting layer 102 containing a material that emits light 107. Within the light-emitting layer 102, light is produced upon electron and hole recombination to generate the light 107. The light-emitting layer 102 may be an inorganic or organic semiconductor layer, or a layer of quantum dots (QDs). At least one hole transport layer 103 is located between the anode 104 and the emissive layer 102, which provides transport of holes from the anode and injection of holes into the emissive layer. Similarly, at least one electron transport layer 101 is located between the cathode 100 and emissive layer 102, which provides transport of electrons from the cathode and injection of electrons into the emissive layer.
FIG. 2 is a drawing depicting the conventional band structure of a conventional light-emitting device as depicted in FIG. 1. FIG. 2 depicts how each layer is important in the transport and recombination of charges and holes in the light-emitting device. In such conventional structures, the layer (or layers) between the cathode and emissive layer (EML) is termed the electron transport layer (ETL), and the layer (or layers) between the anode and the EML is termed the hole transport layer (HTL). The ETL and HTL are collectively referred to more generally as charge transport layers (CTL). The purpose of these CTLs is to provide an ohmic contact to the respective electrode (anode or cathode), and to provide energetic alignment for injecting carriers into the emissive layer.
It is desirable for the HTL to have a highest occupied molecular orbital (HOMO, also referred to as a valence band maximum) that is energetically close to the HOMO (valence band maximum) of the EML to provide efficient hole injection 202. Furthermore, it is desirable for the ETL to have a lowest unoccupied molecular orbital (LUMO, or conduction band minimum) that is energetically close to the LUMO (conduction band minimum) of the EML to provide efficient electron injection 201. Providing efficient and balanced injection of electrons 201 and holes 202 into the EML allows efficient recombination 203 of electrons and holes in the EML, and in turn efficient generation of light 204 from the EML. In this context, balanced injection refers to equal electron and hole current densities being injected into the EML from the ETL and HTL respectively.
An electron injection layer (EIL) or a hole injection layer (HIL) can also be present within the ETL or HTL layers, and is primarily used to aid carrier transfer from the electrode into the adjacent ETL or HTL. An HIL is explicitly present in FIG. 2, as it is common in conventional QD LED structures, but an HIL or EIL may be present within any standard HTL or ETL multi-layers. Specifically, as used herein, an EIL/HIL is a particular kind of ETL/HTL with the specific intention of bridging the energetic gap between the electrode and another ETL/HTL, and thus can be considered an ETL/HTL itself.
Referring back to FIG. 1, in conventional structures the ETL is often comprised of a matrix of nanoparticles 106, which provides electron transport through hopping 105 between adjacent nanoparticles 106 and into the emitting layer 102. Fabricating the device in this way provides a robust ETL, which reduces oxygen and moisture ingress into the EML. Such a configuration, however, has a deficiency in that the ETL has a fixed mobility for a given nanoparticle material configuration. Ideally, to promote radiative (light-emitting) recombination and reduce non-radiative (non-light-emitting) recombination, which occurs more readily when there is an imbalance of electrons and holes in the EML, the mobility of electrons in the ETL should be set at a particular value relative to the mobility of holes in the HTL (or vice versa). For example, it may be preferable for the mobility of electrons in the ETL to be equal to the mobility of holes in the HTL, or an integer multiple thereof, or otherwise matched to maximize radiative recombination.
Conventional processes relating to layers including nanoparticles in quantum dot (QD) light-emitting diodes (QDLED) generally provide conductive paths through an emissive layer, e.g. a mixture of emissive QDs and conductive nanoparticles for improving charge injection into the QDs. See, for example, U.S. Pat. No. 8,361,823 (Kahen, issued Jan. 29, 2013) and U.S. Pat. No. 7,615,800 (Kahen, issued Nov. 10, 2009). Nanoparticle compositions also have been used to provide better sealing of the light-emitting region through the addition of chemically inert nanoparticles. See U.S. Pat. No. 6,838,816 (Su et al, issued Jan. 4, 2005). Conventional processes relating to the nanoparticle charge transfer layers of a QDLED have most commonly tried to tune the mobility by varying the material composition of the ETL layer including homogeneous nanoparticles. See, e.g., KR 101626525B1 (Yang et al., issued Jun. 1, 2016) and CN 106410051 (Zheng et al., published Feb. 15, 2017). Even using such teachings, however, optimization of charge mobility to match ETL electron injection and HTL hole injection has not been achieved.